Are Anonymous Structs Compatible in C17?

• ⚠️ Flattened access with anonymous structs/unions is not guaranteed under C17 and can lead to undefined behavior.
• ✅ GCC and Clang support flattened access as extensions, but they're non-portable and not compliant with strict C17.
• 🔍 C23 formally defines flattened access, improving consistency across compilers and increasing portability.
• ⚠️ Relying on compiler-specific extensions can cause bugs during platform switches or compiler upgrades.
• 📏 Best practices include explicitly naming union members, testing with strict flags, and documenting anonymous usage.

Introduction

You need to know how anonymous structs and unions work under the C17 standard. This helps you write reliable, portable C code. Many embedded systems, older projects, and industrial systems still use C17. Because of this, knowing the limits of flattened access and how compilers handle it matters. It can lead to an easy development cycle, or to many hours debugging strange struct issues. This article explains how C17 deals with anonymous structures and unions. We also compare this to C23, list common problems, and discuss compiler-specific ways of handling them. And then, we will present good practices to help you write better, safer code.

Anonymous Structs & Unions in C17: Basic Concepts

In C, struct and union types group variables under one name. They are basic tools for organizing data. Anonymous structs or unions are declared without a name. They let you access their members directly, so you don't need extra dot notation. This makes the code simpler to write.

Let's take a closer look at how this works in practice.

Named vs. Anonymous Usage

Here's a traditional struct using a named union:

struct Point {
    union {
        int x;
        int y;
    } coords;
};

To access member x, you'd write:

struct Point pt;
pt.coords.x = 5;

Compare that to an anonymous union embedded directly inside the struct:

struct PointAnon {
    union {
        int x;
        int y;
    };
};

Now you can do:

struct PointAnon pt;
pt.x = 5;  // No need for coords

This streamlined access is known as “flattened” access. While it can seem ergonomic and intuitive, its compatibility and safety are highly dependent on the language standard and compiler used.

What Flattened Member Access Really Means

Flattened member access lets you use members of an anonymous structure or union directly. It's like they were declared right in the struct around them.

Benefits of Flattened Access:

• ✅ Gets rid of needless indirection (a.b.c becomes a.c)
• ✅ Makes register mappings or hardware control blocks easier to read
• ✅ Stops you from needing to name things when the meaning is already clear
• ✅ Cuts down on repetitive parts in data structure definitions

Risks Associated with Flattened Access:

• ⚠️ The way it works depends on the compiler
• ⚠️ It makes your code less portable across different toolchains
• ⚠️ You can get unexpected problems when you nest many similar fields
• ⚠️ Stack traces and debug output become harder to understand

Look at this complex example:

struct Nested {
    struct {
        union {
            int a;
            float b;
        };
    };
};

Can you safely use obj.a? Some compilers let you do this. But it does not work everywhere, and the C17 standard does not allow it.

C17's Take on Anonymous Structures and Unions

The C17 standard (officially ISO/IEC 9899:2018) took most of its rules for structs and unions from C11. But it added small, clear updates. It technically allows anonymous structures and unions. Still, this permission comes with limits.

What's explicitly allowed in C17:

• Anonymous unions inside structs
• Anonymous structs inside unions
• Single-level flattened access (this depends on the compiler)

What’s not formally supported:

• Flattening across many levels (for example, an anonymous struct inside another anonymous struct inside a main struct)
• Flattened member access is not guaranteed to work with all tools
• Access methods might not work the same between different compilers or platforms

Here is code that only works where compilers are flexible:

struct Outer {
    union {
        struct {
            int a;
            float b;
        };
        int c;
    };
};

Using outer.a might compile with GCC. But it could fail with MSVC, or during static analysis. This is because C17 has no standard rule for how this pattern should work.

What the Standard Really Says

The ISO C17 specification states:

Unnamed struct/union members can exist as behavior decided by the compiler.

"Decided by the compiler" means the compiler can let this behavior happen or stop it. Your code will not work on different systems if you depend on this.

How C23 Updates the Rules

The new ISO C23 standard changes how anonymous structs and unions work for the better.

C23 Official Behavior:

• ✅ Flattened member access is now a standard feature
• ✅ You can use many levels of anonymous structs/unions
• ✅ Compilers that follow the standard will act the same way

This way of working comes from proposals like WG14 N3000. These proposals want to make things clearer.

Example of Fully-C23-Compatible Code:

struct C23Struct {
    union {
        struct {
            int id;
            float value;
        };
        long timestamp;
    };
};

Accessing id is now explicitly valid in C23 without relying on compiler extensions.

Why This Matters:

• 📦 Your code will work better on different systems
• 🧼 Structures will be cleaner and easier to keep up
• 🚀 New developers can learn faster when they read code that uses structs a lot
• 🧪 There will be fewer hidden bugs caused by different compilers

If a project is already going, or if you are starting a new one, using C23 offers clearer rules for flattened access. These rules are part of the language, not just things compilers happen to support.

Compiler Behavior with Anonymous Structs in C17

C17 says compilers can decide how some features work. Because of this, compilers do not all support anonymous structs the same way.

Example:

struct Test {
    union {
        int x;
        float y;
    };
};

int main() {
    struct Test t;
    t.x = 10;  // ✅ GCC/Clang, ❌ MSVC strict C17
}

Using -std=c17 -Wall -Wpedantic will help identify unsafe accesses that violate the standard.

🔍 (Tip): You can find GCC’s extension documented here. MSVC also works in a similar way because it is compatible with Windows kernel-style coding.

C17 Structs Compatibility Verdict: Is Flattened Access Safe?

Let's be clear: using flattened access with anonymous unions or structs under C17 is not portable. You should not use it in projects that need to work on many systems or in important projects.

✅ It's technically allowed by GCC and Clang as extensions.
❌ It’s not part of the C17 standard.
❌ It breaks under -pedantic or strict-toolchain CI/CD checks.
⚠️ Behavior might change with compiler updates or static analysis tools.

For maximum safety:

• Avoid flattening across multiple anonymous layers.
• Do not assume access is valid unless proven via passing builds in strict modes.

Why This Matters In The Real World

If you use anonymous struct unions when they are not clearly supported, it can cause problems later:

• 🧩 Binary Problems: Flattened access can change how structs are arranged. This can cause issues if you save struct data or send it over a network.
• 🐛 Hard-to-Reproduce Bugs: Compiler upgrades can silently change behavior.
• 🔧 Hard Debugging: Tools like GDB might show fields wrong in stack traces or lists of local variables.
• 📉 Higher Maintenance: People who work on the code later might not understand what flattened fields mean.

If you work with embedded systems, network protocols, or FFI layers (like calling C from Python/Rust), you need to keep struct layouts clear and exact. Flattened anonymous members will not let you do this.

Writing Safe and Compatible Anonymous C17 Unions

Just because something works with your compiler does not mean it is safe. Here is how to write clearer code that works better with different systems.

💡 Use Explicit Naming:

struct Safe {
    union {
        int x;
        float y;
    } coords;  // named, no flattening
};

📌 Isolate Flattening If Necessary:

struct {
    int version;
    union {
        int flag;
        float ratio;
    };  // For use only within this module
} __flattened_internal_config;

✍️ Add Comments & Document Expected Behavior:

struct Config {
    union {
        int id;
        float temp;
    }; // anonymous: only works with GCC
};

Best Practices for Robust Struct Access

Here are ways to make your C17 code strong and reliable:

• ✅ Name All Struct/Union Members – be explicit about structure
• ✅ Use Static Assertions – to check where fields are placed in memory
• ✅ Wrap Unsafe Macros – like so:

#define ACCESS_ID(x) ((x).u.id)

• ✅ Use Compiler Flags for Safety:

gcc -std=c17 -Wall -Wpedantic -Werror

• ✅ Guard for Custom Behavior:

#if __STDC_VERSION__ >= 202311L
// Use C23
#else
// Use safer alternative
#endif

Looking Ahead: Upgrade to C23?

Moving to C23 will make your code easier to keep up with later. Think about these points:

• 🎯 Does your compiler support C23? (e.g., GCC 13+ or Clang 16+)
• 🛠 Are critical tools (linters, IDEs, analyzers) compatible?
• 📈 Will clearer rules for structures help your team work faster?

If you answer yes, then the clearer rules for flattened member access alone make the change worthwhile.

Team-Level Practices for Safer Struct Use

For teams that share and maintain C code, setting rules and policies is very important.

👥 Suggested Team Policies:

• Have code rules that advise against anonymous access in C17
• Use CI pipelines that run strict compiler warnings
• Add checks for struct use to pull request reviews and code review guides
• Document all flattened member dependencies and intended behaviors
• Use unit tests that verify expected memory layouts or field access

In Summary: Prioritize Clarity and Compatibility

Anonymous structures and unions might seem like quick fixes. But under C17, they can lead to bugs that are hard to find and problems with code not working on different systems. Until you can fully use C23 in your project, use clear, named members. Also, test any flattened access very carefully. Writing code that follows rules, is clear, and works on many systems helps keep it easy to maintain. And it stops bad structure bugs from showing up in your future releases.

Citations

• ISO/IEC. (2018). ISO/IEC 9899:2018 – Programming Languages — C. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf
• ISO/IEC JTC1/SC22/WG14. (2022). WG14 N3000: Working Draft for C23 Standard. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3000.pdf
• GNU Compiler Collection. (n.d.). GCC Manual – Extensions to the C Language Family. https://gcc.gnu.org/onlinedocs/gcc/Unnamed-Structure-and-Union-Fields.html
• Developers’ Discussion on Compiler Behavior. (2023). Stack Overflow / Developer Forums. https://stackoverflow.com/questions/74584698/is-it-safe-to-use-anonymous-unions-in-c17

Do you want to update your C code or make sure it works for a long time? Start by checking how you use structs and unions. Your future self (and your coworkers) will be glad you did.